Crystalline molecular sieves all have a 3-dimensional, four-connected framework structure of corner-sharing [TO4] tetrahedra, where T is one or more tetrahedrally coordinated cations. Examples of well known molecular sieves are silicates, which comprise [SiO4] tetrahedral units; aluminosilicates, which comprise [SiO4] and [AlO4] tetrahedral units; aluminophosphates, which comprise [AlO4] and [PO4] tetrahedral units; and silicoaluminophosphates, which comprise [SiO4], [AlO4], and [PO4] tetrahedral units.
Molecular sieves are typically described in terms of the size of the ring that defines a pore, where the size is based on the number of T atoms in the ring. Other framework-type characteristics include the arrangement of rings that form a cage, and when present, the dimension of channels, and the spaces between the cages. See van Bekkum, et al., Introduction to Zeolite Science and Practice, Second Completely Revised and Expanded Edition, Volume 137, pp. 1-67, Elsevier Science, B. V., Amsterdam, Netherlands (2001). For example, zeolite and zeolite-like molecular sieves are microporous materials containing pores and cavities having a size that range from a few angstroms to about 2 nanometers. In this application, the terms “micropore”, “microporous,” and all their derivatives refer collectively to pores having a diameter of less than 2 nanometers.
Zeolites and zeolite-like materials are characterized by their chemical composition (e.g., Si:Al atomic or molar ratios), as well as their crystal framework connectivity, conveniently described by a topological model. For a given chemical composition, an infinite number of theoretical structures is possible. Zeolites with over 130 different topologies have been synthesized, characterized and assigned a three letter code as mentioned in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001).
Zeolites and zeolite-type materials are widely used in separation processes (ion exchange, selective sorption). In their acid form, zeolites and zeolite-like materials are acid catalysts, due to the combination of their strong acidity and molecular size- and shape-selectivity. Such catalytic reactions normally take place in the pores and cavities of zeolites and zeolite-type materials but intra-particle diffusion limitations and pore blocking can prevent accessibility to a large number of catalytic sites.
One way to improve the diffusion properties of a molecular sieve is to reduce the size of the molecular sieve crystals. Various methods have been described to make small crystal size molecular sieves (see, for example, International Patent Publication Nos. WO 00/06492, WO 00/06493, and WO 00/06494). However, the colloidal behavior of very small particles makes them difficult to recover and handle, especially on an industrial scale. Moreover, reducing crystal size does not address the issue of diffusion within the molecular sieve crystals.
Diffusion within molecular sieve crystals can be enhanced by the inclusion of intacrystalline mesopores, which can act as “molecular highways” to and from the active sites of the sieve thereby reducing the average intrapore diffusion distance from, for example, 1 micron, to, for example, 50 nanometers or less. This can, of course, be achieved by synthesis of molecular sieves having uniformly distributed pores all sized within the mesoporous range, such as the MCM-41 materials reported by Kresge et al., in Nature, 1992, 359, 710 and by Beck et al., in J. Am. Chem. Soc. 1992, 114, 10834. In the context of the present invention, the terms “mesopore”, “mesoporous,” and all their derivatives refer collectively to pores having a diameter of from 2 to 50 nanometers.
However, while such mesoporous materials offer good diffusion properties, they frequently lack the strong acidity of their microporous analogues, and, of course, the desirable shape selectivity of microporous zeolite and zeolite-like materials is lost. Therefore, various strategies have been developed to modify the physical and chemical properties of mesoporous materials. For example, U.S. Pat. No. 5,145,816 discloses post-synthesis functionalization of MCM-41 type materials. In addition, it is known to encapsulate metal oxides in the mesopores of MCM-41 materials, see, for example, Dapurkar et al., Catalysis Today, 68 (2001), pp. 63-68.
Various methods have also been proposed for producing molecular sieves that exhibit both microporosity and mesoporosity. For example, U.S. Pat. No. 6,358,486 describes a process for producing an inorganic oxide, such as a silicate or aluminosilicate, that contains micro- and mesopores, comprising heating a mixture comprising water, an inorganic oxide and at least one compound, such as a glycol, that binds to the inorganic oxide by hydrogen bonding. Typically, the mixture also includes a template of the type that is used for producing micropores in zeolite synthesis, such as tetramethylamonium, tetraethylammonium, tetrapropylammonium, and tetrabutylammonium salts.
In addition, U.S. Pat. No. 6,843,977 describes a porous structured aluminosilicate composition which comprises a framework of linked tetrahedral SiO4 and AlO4 units assembled from zeolite fragments, the framework defining pores having an average size of 1 to 100 nanometers and a Si to Al molar ratio of between about 1000 to 1 and 1 to 1, wherein the composition has at least one x-ray diffraction peak between 2 and 100 nm and retains at least 50% of its initial framework pore volume after exposure to 20 volume % steam at 800° C. for two hours. The composition is produced by assembly of a hexagonal aluminosilicate structure from seeds that would normally nucleate the crystallization of zeolite Y, ZSM-5, or zeolite beta. The seeds are heated in the presence of a surfactant, such as cetyltrimethylammonium bromide or a non-ionic PEO block copolymer, to form the mesoporous hexagonal structure.
U.S. Published Patent Application No 2001/0003117, published Jun. 7, 2001, discloses a method of preparing zeolite single crystals comprising the step of applying a synthesis gel with a zeolite precursor composition within the pore system and on the surface of a particulate matrix material having a predetermined pore structure and particle size; subjecting the precursor composition to crystallizing conditions; and isolating porous single crystals of the zeolite by removing the matrix material. The matrix material is preferably carbon black, which can be removed by controlled combustion or hydrogenation to create mesopores inside the individual large crystals.
To date, only silicates and aluminosilicates having both microporosity and mesoporosity have been reported; there have been no reports of the synthesis of aluminophosphates and silicoaluminophosphates with such bimodal pore distribution. According to the invention, it has now been found that certain microporous aluminophosphate and silicoaluminophosphate molecular sieves, in particular, those having the CHA framework type, having intracrystalline mesopores can be synthesized by growing the crystalline material around nanosized particles of a thermally decomposable material, such as carbon black.